The present invention relates to methods of controlling proliferation and differentiation of stem and progenitor cells. In one aspect, the present invention relates to a method of imposing proliferation yet restricting differentiation of stem and progenitor cells by treating the cells with chelators of transitional metals, resulting in reduction in transitional metals availability. In another aspect, the present invention relates to a method of inducing differentiation of cells by treating the cells with chelators of transitional metals, resulting in increase in transitional metals availability to the cells. In still another aspect, the present invention relates to assays for determining whether a specific chelator of transitional metals will restrict or induce differentiation.
Cell Differentiation and Proliferation
Normal production of blood cells (hematopoiesis) and of other cell types involves the processes of proliferation and differentiation which are tightly coupled. In most hematopoietic cells following division the daughter cells undergo a series of progressive changes which eventually culminate in fully differentiated (mature), functional blood cells, which in most part are devoid of proliferative potential. Thus, the process of differentiation limits, and eventually halts cell division. Only in a small minority of the hematopoietic cells, known as stem cells, cell division may result in progeny which are similar or identical to their parental cells. This type of cell division, known as self-renewal, is an inherent property of stem cells and helps to maintain a small pool of stem cells in their most undifferentiated state. Some stem cells lose their self-renewal capacity and following cell division differentiate into various types of lineage committed progenitors which finally give rise to mature cells. While the latter provide the functional capacity of the blood cell system, the stem cells are responsible for the maintaining of hematopoiesis throughout life despite a continuous loss of the more differentiated cells through apoptosis (programmed cell death) and/or active removal of aging mature cells by the reticuloendothelial system. It will be appreciated that in one way or another these processes characterize all other cell lineages of multicellular organisms, because replenishment of dead cells occurs during the life cycle of such organisms.
Normal hematopoiesis is coordinated by a variety of regulators which include glycoproteins such as the colony stimulating factors (CSF), as well as small molecules such as the retinoids. They regulate the survival (e.g., by inhibiting apoptosis), proliferation and differentiation of progenitor and precursor cells and the activation state of mature cells.
In acute leukemia, for example, there is a block in cell differentiation. As a results, the leukemic cells maintain their proliferative potential. Leukemic cells do not respond normally to the various regulators (54). Thus, cells obtained from patients with acute myeloid leukemia develop in culture, in response to stimulation by colony stimulating factor (CSF), small colonies of undifferentiated cells, as compared to large colonies of granulocytes and macrophages, which develop following cloning normal hematopoietic cells.
As further detailed below, expansion of the stem cell and other defined lympho-hematopoietic cell subpopulations by ex-vivo culturing could have important clinical applications.
A variety of protocols have been suggested and experimented for enrichment of such populations. The main experimental strategies employed include incubation of mononuclear cells with or without selection of CD34+(8); with different cocktails of early and late growth factors (17); with or without serum (7); in stationary cultures, rapid medium exchanged cultures (18) or under continuous perfusion (bioreactors) (6); and with or without established stromal cell layer(19).
Although a significant expansion of intermediate and late progenitors was often obtained during 7–14 days ex-vivo cultures, the magnitude of early hematopoietic (CD34+CD38−) stem cells with high proliferative potential, usually declined (6, 20–22).
Thus, these cultures do not result in true stem cell expansion, but rather in proliferation and differentiation of the stem cells into pre-progenitor cells, accompanied by depletion of the primitive stem cell pool.
In order to achieve maximal ex-vivo expansion of stem cells the following conditions should be fulfilled: (i) differentiation should be reversibly inhibited or delayed and (ii) self-renewal should be maximally prolonged.
Similarly, following cell expansion, it is important to have methods to induce differentiation of the expanded cell population, so as to covert the expanded cell population to mature functional cells or tissue.
Role of Copper in Cell Differentiation:
The possible involvement of Copper in hematopoietic cell development could be inferred from the following findings:
Clinical symptoms in Copper deficiency: Copper deficiency can result from hereditary defects, such as Menkes syndrome or Celiac disease, or from acquired conditions. The latter is typically associated with malnourishment. It may be caused by Copper non-supplemented total parenteral nutrition (e.g., following intestinal resection), by consumption of high levels of Zinc, which interferes with Copper utilization, in underweight and/or cow milk (poor source of Copper) fed new-borns, which may result in severe cases in Shwanchman syndrome. Unbalanced treatment with Copper chelators in Copper overload cases such as in Wilson's disease may also lead to Copper deficiency.
The clinical symptoms of Copper deficiency may include impairment of growth, brain development, bone strength and morphology, myocardial contractility, cholesterol and glucose metabolism, host defence (immune) mechanisms and more.
Of particular relevance to this study is the fact that Copper deficiency is often associated with hematological abnormalities, including anemia, neutropenia and thrombocytopenia. All these pathological manifestations are unresponsive to iron therapy, but are rapidly reversed following Copper supplementation (27–28).
The mechanism by which Copper deficiency leads to neutropenia is unknown. Among the possible causes, either alone or in combination, are: (i) early death of progenitor cells in the bone marrow (BM); (ii) impaired formation of neutrophils from progenitor cells in the BM; (iii) decrease in cellular maturation rate in the BM; (iv) impaired release of neutrophils from the BM to the circulation; (v) enhanced elimination rate of circulating neutrophils.
Examination of the BM of neutropenic Copper-deficient patients demonstrates the absence of mature cells (“maturation arrest”). It has been shown that cells derived from such BM did not form colonies in semi-solid medium containing Copper deficient serum, but retained the potential for normal colony growth in Copper containing serum. These results indicate the presence of intact progenitors in the patient's BM, and suggest that the block in development occurs distal to the progenitor stage (29–30).
The effect of Copper in cell lines: The effect of Copper was also studied in-vitro established cell lines (31–34). One such line (HL-60) was derived from a patient with acute promyelocytic leukemia. These cells, that have the characteristics of myeloblasts and promyclocytes, can grow indefinitely in culture. Upon addition of various agents, such as retinoic acid (RA), to the culture medium, the cells undergo differentiation, which results in cells which demonstrate some, but not all, features of mature granulocytes.
The study of Copper status in these cells has shown that although the cytosolic Copper content per cell was not significantly different in RA-treated cells compared to untreated cells, the Copper content per protein content was doubled. This is due to the fact that RA-treated cells have about half the protein content as compared to their untreated counterpart. Using 67Cu, it has been shown that the rate of Copper uptake was significantly faster during the two first days of RA treatment, but not at later times. The intracellular distribution of 67Cu was found predominantly in high molecular weight (MW) fractions (>100 kD) and a lower MW fraction of about 20 kD, with a higher proportion of Copper present in the high MW fractions in RA-treated cells.
Addition of excess Copper to regular serum-supplemented growth medium modestly increased RA-induced differentiation. Although RA-treated HL-60 cells do not necessarily represent normal cell development, these results point to the possibility that neutrophilic differentiation may require Copper.
In other experiments it has been shown that HL-60 cells can be made Copper deficient by treatment with Copper chelators, and that following such treatment their viability and growth rate were unaffected.
Although all these phenomena have been attributed to Copper, it has been reported that some clinical and biological effects are shared by Copper and other transition metals:
For example, clinical symptoms similar to those observed in Copper-deficiency could also be observed following consumption of high levels of Zinc (40–42), which has been known to interfere with Copper utilization (e.g., 43).
In a study of human hepatocellular carcinoma it was found that the concentrations of both Copper and Zinc in the tumor tissue decreased with the degree of histological differentiation (44).
In another study it was shown that addition of Copper, Zinc and Ferrum to primary cultures of rat hepatocytes induced cell replication and formation of duct-like structures. The cells lining the ducts became morphologically and biochemically characteristic of bile duct cells (45).
Various transition metals are known to influence the production and activities of many enzymes and transcription factors associated with differentiation. Examples include the Cu/Zn containing superoxide dismutase (46); the metallothioneins and their transcription regulating factors (e.g., MTF-1) (47–49); the 70 kDa heat shock protein (hsp70) (50); the p62 protein which associates with the ras-GTPase activating protein during keratinocyte differentiation (51); a neutral sphingomyelinase which is activated during induced differentiation of HL-60 cells (52); and the bovine lens leucine aminopeptidase (53).
Oligopeptides, either natural or synthetic, can bind Copper too. Thus, glycyl-L-histidyl-L-lysine-Cu2+ (GHL-Cu) is a tripeptide-Copper complex that was isolated from human plasma. It has been shown to have, in nanomolar concentrations, a variety of biological effects both in-vitro and in-vivo: It was first described as a growth factor for a variety of differentiated cells (55). Subsequent data from various groups indicated that it exhibited several properties of a potent activator of the wound healing process. It was a potent chemotactic agent for monocytes/macrophages and mast cells (56–57). It stimulated nerve tissue regeneration (58) and was reported to trigger the angiogenesis process in-vivo (59). It stimulated collagen synthesis in several fibroblast strains (60). It accelerated wound closure when injected into superficial wounds in animals (61–62) and accumulation of collagen and dermatan sulfate proteoglycans (63). It also exerted metabolic effects, such as inhibition of lipid peroxidation by feritin (64). GHL-metal ions combinations were shown to promote monolayer formation and cellular adhesiveness in tumorigenic hepatoma (HTC4) cells in culture, resulting in marked enhancement of cell survival and growth under basal (growth limiting) conditions (65). The mode of action of GHL is unknown. It has been reported that GHL forms chelates with Copper and iron in human plasma and in buffered solution at physiological pH.
While reducing the present invention to practice, it was found that a series of chemical agents that bind (chelate) transition metals, Copper in particular, can inhibit (delay) the process of differentiation of stem cells as well as intermediate and late progenitor cells and thereby stimulate and prolong the phase of active cell proliferation and expansion ex-vivo. This newly discovered effect of Copper and other transition metals depletion (either partial or complete depletion) was used for maximizing the ex-vivo expansion of various types of cells as further detailed hereinunder. However, it was also found, while reducing the present invention to practice, that a series of other transition metal chelators, Copper chelators in particular, can induce the process of differentiation in cells, e.g., both normal and leukemic hematopoietic cells ex-vivo.